by S. Prasad Ganti
Every year between January and March, there is a series of “Science on Saturday” lectures at the Princeton Plasma Physics Laboratory. These lectures have been virtual for the past two years due to the pandemic. Nevertheless, the lectures are recorded and are available in the archives. One such lecture caught my attention this year. It is titled “Feeling the Heat: Fusion Plasmas Used to Study Spacecraft Heat Shields”. The link given below has the complete lecture. I am summarizing at a high level.
Taking the heat! Entry, descent and landing payloads on atmospheric bodies require special materials, Large Heat Shield for Mars Science Laboratory, Photo Credits: NASA
Let us start with plasma. It is the fourth state of matter after solid, liquid, and gas. As the temperature increases, the state of a substance changes from solid to liquid to gas and then to plasma. So plasma is very hot indeed and can be found wherever intense heat is there like in the sun, the stars, the nuclear fusion reactors etc. In fact, it is said that 99% of the visible universe is plasma. Visibility comes from the stars anyway.
On the earth, we have neon lights which contain plasma when they glow. Similarly, the northern lights or the southern lights at the poles result from heating up of earth’s atmosphere by charged particles coming from the sun. Fusion reactors are mainly engineering structures used to generate energy by creating a sun on the earth in a magnetic bottle. They are largely experimental and the goal is to move towards generating energy on industrial scales.
In the sun and the stars, such huge temperatures are generated by gravitational compression of gasses like hydrogen wherein it fuses together to form helium. In the process, it releases huge amounts of energy in the form of light and heat. In the fusion reactors, heat is supplied by electricity to fuse together the hydrogen. The expectation is that the fusion reaction will generate much more electricity than what it consumes.
Whenever a spacecraft returns to earth’s dense atmosphere from space or enters the dense atmosphere of another planet like Mars or Jupiter, intense heat is generated. Mainly because spacecraft travel very fast in space. Like tens of thousands of miles per hour. Such speeds when encountering dense atmospheres, heat up the surrounding air and create a layer of plasma.
Heat shields are designed and placed in front of spacecraft to absorb such heat from the surrounding plasma and protect the structure and the contents of the spacecraft. These heat shields take the shape of a cone on the nose of the spacecraft or the thousands of tiles lining up the front part of the now retired space shuttle. These shields are made of specific materials to absorb the heat and are engineered very carefully.
Since we have plasma in the fusion reactors on the earth, studies are being done on how specific heat shields behave in the presence of plasma. Fusion reactors are being used as design tools for the heat shields. To determine which materials hold up well and which shapes or the structures are better suited for the job. This is better than guesswork and learning from how the heat shields function on re-entries. It will give us a chance to know exactly how a particular heat shield will perform.
Designing heat shields is a very good application of the fusion reactors. It does not disturb the main functioning of the fusion reactor. Just a minor placement of the material to be studied in a small corner of the reactor and making observations. We can expect better heat shields for future space missions. The number of space missions will only increase in the future and optimizing heat shields will be an important milestone in such a journey.